Zero arc drop thyratron

ABSTRACT

A thyratron has a tubular cathode, preferably made of rhenium and circumscribed by a grid which in turn is circumscribed by an anode tube. The thyratron cavity is filled, e.g., with vapor pressure regulated cesium. The operating parameters can be chosen to establish essentially zero voltage drop across the thyratron. The cathode cavity is contiguous with a cavity of a thermionic converter.

United States Patent lnventor William J. Keams Arcadia, Calif. Appl. No.644,146 Filed June 7, 1967 Patented May 18, 1971 Assignee XeroxCorporation ZERO ARC DROP THYRATRON 18 Claims, 7 Drawing Figs.

U.S. Cl 315/363, 313/40, 313/180, 313/186, 313/211, 313/218, 313/227Int. Cl. ..l-l0lj 17/08, l-lOlj 17/20 Field of Search 313/218,

Primary Examiner.lames W. Lawrence Assistant ExaminerPalmer C. DemeoAttorneySmyth, Roston & Pavitt ABSTRACT: A thyratron has a tubularcathode, preferably made of rhenium and circumscribed by a grid which inturn is circumscribed by an anode tube. The thyratron cavity is filled,e.g., with vapor pressure regulated cesium. The operating parameters canbe chosen to establish essentially zero voltage drop across thethyratron. The cathode cavity is contiguous with a cavity of athermionic converter.

74' erna/fire yy fade fn/t/F/tC/fd/t 3 Sheets-Sheet z Patented May 18,1971 ZERO ARC DROP THYRATRON The present invention relates to athyratron for high temperature operation and is adapted for zero voltagedrop between anode and cathode electrodes. One of the principleoperating features of thyratrons is a high thermionic emission of thecathode, but low thermionic emission of the anode. The high thermionicemission of the cathode can best be obtained by operating the cathode ata high temperature and by selecting a material which the so-called workfunction is very low. High temperature and low work function ensureshigh thermionic emission of any particular material where Richardsonslaw is applicable.

A low cathode work function and a high anode work function makes itinherently impossible to operate a thyratron with zero voltage dropbetween cathode and anode electrodes because the polarity of thedifference of the work function is then such that the internal voltagedrops in the thyratron have the same sign and thus add to the workfunction difference, In order to obtain zero voltage drop between anodeand cathode of a thyratron it is, therefore, essential to have thereverse relationship of work functions, i.e., the cathode work functionmust be high and the anode work function must be low. From thestandpoint of thermionic emission this appears to be an undesirablecondition for thyratron operation. Of course, a suitable temperaturedifferential between anode and cathode can still produce a highthermionic emission of the cathode and a low thermionic emission of theanode even though the work functions are selected that they do not aidin this relationship.

It has been found now that thermionic emission of a material with a highwork function can be increased (for any given temperature) if the gas inthe discharge chamber of the thyratron has a very low work function anda low ionization voltage and, therefore, adheres to the cathode even atcathode temperatures well above the boiling point of the gas. As aresult of this adhesion the effective thermionic emission of the cathodeis materially increased, In such a situation the thermionic emissions ofcathode and anode are not determined any more in the simple mannerfollowing the Richardsons Law, but the gas or vapor modifies thethermionic emission of anode and cathode to a substantial degree and inproportion to the adhesibility of the gas or vapor molecules, whichcharacteristics is in turn predominantly determined by the work functiondifference of the cathode material on one hand, and of the gas orvapor,on the other hand.

In the following it appears to be convenient to speak of the workfunction of a gas or vapor even though such terminology is not exactlycorrect. What is meant then is the work function of that material whenin the liquid or solid state. Even though vapor and gas molecules adhereto a surface such as a cathode surface having a temperature well abovethe boiling point of that gas or vapor material, no true liquid or solidstate of that gas or vapor material is established at the hot surface,but the adhering molecules define a surface strata which modifies thethermionic emission of the substrata and the work function thereof. Onlywhen the surface has obtained substantial dimensions at rather lowtemperatures, then the work function of the strata itself issubstantially that of the vapor or gas material when in the liquid orsolid state.

If the anode is kept relatively cool its effective work function will besuperseded by the bulk work function of the vapor material allowed toprecipitate to a material extent on the anode. If, in addition, thevapor has a low ionization voltage, the total internal voltage drop inthe thyratron can in fact be reduced to the effective work functiondifference between anode and cathode. And the result is a zero orsubstantial zero voltage drop measured externally between anode andcathode of the thyratron.

In particular the internal voltage drop in the thyratron depends on theplasma drop and the voltage drops in anode and cathode sheaths. Theselatter sheath drops in turn depend on the ionization voltage of thevapor which has to be rather low. Cesium and rubidium are highlybeneficial for this purpose, but all alkaline vapors can actually beused, at least to the extent that the voltage drop across the thyratroncan be made rather low.

The plasma drop depends primarily on the gas pressure and the electrodespacing in accordance with Paschens pd law for established plasmas. Aseparate component of the plasma drop is the grid drop, which is usuallyconsidered separately. This grip drop results from the constriction ofthe electric current path between anode and cathode, by the grid of thethyratron. It has been found possible to select the aperture of the gridwide enough without losing control so that that grid voltage is indeedalso small. An important aspect is that for establishing zero voltageconditions, the cathode work function itself, and even the differencebetween cathode and anode work functions is the largest single potentialdifference in the entire anode-plasma-cathode system and enters into theconsideration as modified by the adhesion of the plasma atoms such ascesium atoms and thus becomes subject to control and selection inaccordance with existing operating conditions. It follows that thecathode work function must be relatively high. A high work function is awork function having value comparable with the work functions ofrhenium, molybdenum, tantalum, etc. Cesium and rubidium have typicallylow work functions.

A large temperature differential between anode and cathode is beneficialto the operation of the thyratron. A high cathode temperature can, e.g.,be obtained when the cathode is a cylindrical tube with grid and anodebeing concentrically disposed about the cathode tube. The interior ofthe cathode tube fonns a radiation cavity which is exposed, e.g., tothermal radiation. In the preferred form of practicing the invention,this cavity is an extension of a cavity defined by thermionic converterswhich convert thermal radiation energy into the electrical energy usedfor powering the thyratron or several thereof. This kind of arrangementis, e.g., highly useful as a power supply source in a space vehicle inwhich reflectors direct solar radiation into the above defined cavitiesfor operating the thermionic converters and thermally biasing andenergizing the thyratrons. These thyratrons may then pertain to aninverter which converts DC electric energy as provided by the thermionicgenerators into a suitable AC voltage and current.

While this specification concludes with claims particularly pointing outand distinctly claiming the subject matter which is regarded as theinvention, it is believed that the invention, the objects and featuresof the invention and further objects, features and advantages thereofwill be better understood from the following description taken inconnection with the accompanying drawings in which:

FIG. 1 illustrates somewhat schematically a power supply system;

FIG. 2 illustrates in perspective view, partially broken open withsection view of a thyratron;

FIGS. 3a, 3b and 3c illustrate relevant characteristics for thethyratron shown in FIG. 2;

FIG. 4 illustrates a top view of the thyratron shown in FIG. 2; and

FIG. 5 illustrates a section view of a different thyratron in accordancewith the present invention.

Proceeding now to a detailed description of the drawing in FIG. 1thereof, there is shown the general layout of the novel conversionsystem, particularly the high temperature section thereof. As was statedabove, this system may serve as a power supply system for a spacevehicle using solar energy. Radiation enters the system, in the drawing,from the left. This radiation may have been focused by a large concavereflector (not shown) which reflector is oriented towards the sun toreceive solar radiation. That reflector may be positioned to the rightof the system as illustrated in FIG. 1. A second smaller reflector, alsonot shown, may be disposed in the focal area of the larger and firstmentioned reflector to direct light now as in dicated as radiationparticularly towards and into a cavity 15 of a thermionic conversionsystem 10.

The cavity 15 of the system is coaxial with a second cavity pertainingto two coaxially positioned, ring-shaped thyratrons 20 and 30. Thethyratrons pertain to an electric inverter which includes a transformer40. The secondary winding 41 of the transformer leads to what can bedescribed as a high temperature-low temperature interface 50 separatingthe high temperature unit 10-20-30-40 from other elements placed at asufficient distance therefrom to be maintained safely at lowertemperatures.

The low temperature unit may include particularly a rectifier 51 and allcircuit elements driven by the output of the rectifier 51. The rectifiercompletes the system as a DC-AC-DC conversion system. The circuitelements connected to the rectifier 51 are in particular all instrumentsand electrically powered components in the space vehicle to which thisunit pertains. These other elements are summarily denoted with numeral52 and do not pertain to the invention proper. The low temperature unitincludes also a circuit network 60 for driving the grids of thethyratrons. This circuit network 60 includes a voltage regulator 61connected to the rectifier 51 and driving an astable multivibrator 62which in turn drives a switching circuit such as a bistablemultivibrator 63. lndividual output amplifier stages 64 and 65 lead fromthe multivibrator 63 to the grids of the thyratrons 20 and 30 throughsuitable connections. Since the grid current can be kept quite low thecircuit connections between the control circuits and the grids of thethyratrons do not impose any weight problem nor are there considerablelosses. In addition these wire connections can be included in the seriesimpedance for the grid circuit which is usually necessary for thethyratron grids.

The thermionic converters are not themselves subject matter of thepresent invention, but they are the principal electric power source andshall thus be described briefly. There are four units 11, l2, l3 and 14,(not shown), each having an emitter and the four emitters are arrangedso that their outer contour defines the cavity 15. Thus, the fourconverter units are radially disposed around the cavity and theirrespective collectors face in outward direction.

The converter units are electrically connected in series whichconnection includes the connectors 16 and 17 illustrated extendingbetween the collector of converter 11 and emitter of converter 12(connector 16), and between collector of converter 12 and emitter ofconverter 13 (connector 17). The radiation boils electrons from thesurface of the emitters in the interior of the converters; the emittersare made of, e.g., rhenium. These electrons are collected by therespective collectors, lowering the potential thereof. Each converterunit will yield a voltage of about 0.04 to 0.7 volts so that the totalyields of the four units when connected in series may be 1.5 to 3 voltsat about 50 amperes. This is in the order of 10 watts. The temperatureof the emitter electrodes and thus of the cavity 15 is raised up to 1700C. or above. That temperature is needed to operate the thermionicconverters at sufficient efficiency. Of course, the cavity 15communicates with the vacuum of outer space. The system has two outputbus bars, one of them is connector 18 and leads from the collector ofconverter 13 to the center tap of the primary winding 42 of transformer40. The other output bus bar is not shown, but it leads from the fourconverter (14) to the cathode of the thyratrons 20 and 30.

These thermionic converters yield only a rather low voltage at highcurrent. ln'order to make efficient use of this electrical energy it isessential that it be converted into a high voltage at correspondinglylower current in order to avoid long and thick connectors for a highcurrent. Such connectors could be heavy, or, if thin, they would belossy. Both features are undesirable in space vehicles where weight mustbe kept low and electrical energy must not be wasted. Thus, the electricconversion DC-AC unit 20, and 40 should be very closely positioned tothe thermionic conversion unit 10 producing the high current-low voltageDC in order to avoid long output connectors for the thermionic unit. Itfollows, therefore, that the electrical conversion unit must be operatedat high temperatures.

FIG. 2 illustrates a first embodiment for the two thyratrons designed tooperate at high temperatures, whereby the high operating temperature isof advantage for efficient thyratron operation. The two thyratrons areconstructed similarly so that details are shown only for one thyratronof this particular embodiment.

The thyratron 20 is comprised of a cylindrical or cathode tube 21 fromwhich extend two supporting rings or annuli 211 and 213 havingrespectively two ring-shaped grooves to provide heat chokes 212 and 214.The two rings 21] and 213 serve as mounting media for the otherelectrodes, and the heat chokes 212 and 214 provide some thennalinsulation between the cathode proper and the portions of rings 211 and214 used for mounting the other electrodes.

The cathode tube with annuli could be made of molybdenum or tungsten,tantalum or niobium; preferably, however, it is made of rhenium. Thecathode tube 21 with annuli 211 and 213 defines an annulus having adouble U-shaped cross section wherein the entire interior of the U is aring-shaped discharge chamber of the thyratron which, by itself, is anannulus. The principal cylindrically-shaped surface of the cathodeemitting electrons is denoted with reference numeral 215. The emissionoccurs thus principally in radial, outward direction.

The anode of the thyratron is formed by an annulus 22 having a T-shapedcross section whereby the crossbar of the T defines the tubular-shapedanode proper, 221. The stem of the T alternates in length around thecircumference of the annulus 22 to define a plurality of, e.g., threeradiator vanes 222, 223 and 224. These vanes are the single coolingmeans for the anode.

The structure should be mounted to the space vehicle so that theradiators are exposed directly to the vacuum of outer space and so thatother parts of the vehicles are not exposed to the thermal radiationemanating from the radiators nor should conduction be possible to thelow temperature section of the vehicle. Cooling occurs thus solely bythe emission of radiation. It will be appreciated that this provides forthe hardest conceivable environment for operation of the thyratron. In adifferent environment a more efficient cooling may be available so thatthe operation can be improved to that extent.

The anode is made of any of the materials mentioned above for use ascathodes. For reasons below the anode material is of lesser importance.Two flat supporting rings or annuli 225 and 226 are hermetically joinedto the crossbar of the anode T and extend parallel to the stem thereofas well as to the radiators 222, etc., and form sealing surfaces forjoining the anode to the insulator rings 252, 253.

The grid of the thyratron is composed of two separate annuli 23 and 24,each of them having a double L-shaped cross section. The long leg ofeach L is a flat ring structure, 234 and 244 respectively. From eachring there extend, in the same direction, three radiator vanes forcooling the grid. Thus, there are three radiator vanes 231, 232 and 233respectively, extending from the ring 234 of the grid element 23 andthree corresponding radiator vanes 241, 242 and 243, respectively,extend from ring 244 of the element 24. The short legs of the two L'sdefine two coaxial grid tubes 235 and 245 respectively and each havingnarrow axial dimensions. The two axial grid tubes 235 and 245 arepositioned to define a circumferential passage 25 between cathodesurface 215 and anode surface 221.

The grid ring 234 is sandwiched in between two ceramic rings 251 and 252by means of which this grid element 23 is mounted in between the twoannuli 211 and 225, respectively, pertaining to cathode and anodestructure. Analogously the grid ring 244 is sandwiched in between twoceramic rings 253 and 254, by means of which this second grid element 24is mounted in between the annuli 213 and 226, respectively, pertainingto cathode and anode structure. The rings 25] to 254 can be made of anyinsulating material capable of withstanding the high operatingtemperatures contemplated; ceramic is the most suitable material here.

If space permits, a heat shield is interposed between the cathode andthe grid. The heat shield is composed of two cylindrical or tubularelements 261 and 262. The aperture space defined between them may besomewhat narrower than the grid gap 25, to provide sufficient shieldingfor the grid and for the anode from the thermal radiation emanatingparticularly from the cathode. The shield elements can be made of anymaterial capable of withstanding the high temperature encountered in thethyratron, and, further, they must be capable of bonding to the rings211 and 213. In order to eliminate the latter problem, the shieldelements can thus be made of the same material chosen for the cathode.

The interior space of the thyratron is filled with cesium vapor. Thereis provided a cesium reservoir 27 which communicates with the interiorof the thyratron through a bore 271 in one of the anode heat dissipaterse.g., the radiator 224. The bore 271 may be a capillary so that thecesium in the liquid state cannot flow into the interior of thethyratron. Thus, the reservoir is of the O-g type. The reservoir 27 islocated sufficiently far from any of the surfaces facing the interior ofthe thyratron, so that the temperature of the reservoir can be kept wellbelow the boiling point of cesium, which is 670 C. for atmosphericpressure. The outer portions of the radiator vanes are sufficiently coolso that the reservoir can be kept at 300 C. for which a very lowpressure equilibrium can be established in the interior of thethyratron.

Even though the thyratron chamber proper has rather hot walls, thepressure in the interior of the thyratron is determined by the coolestpoint in communication with the vapor and this is the reservoir. Such alow temperature is needed because the pressure in the thyratron is wellbelow the atmospheric pressure under the desired operating conditions.For reasons below, the vapor pressure is in the range of 1 Torr orbelow. For such low vapor pressures, very little heat is transferred byand through the vapor. A quantitative analysis revealed that the vaporparticipates only very little in the heat transport from the cathode tothe other electrodes so that the heat content of the vapor is also verylow. The temperature of the reservoir is such that a heater 272 for thereservoir can be used to regulate the vapor pressure in the thyratron.

The thyratrons and 30 have the following electric circuit connections.The anode output lead is a bar 281 which is welded or brazed to one ofthe anode radiators, e.g., radiator 224. The bar 281 leads to one sideof the primary 42 of the transformer 40. The cathode tube 21 of thethyratron is in intimate contact with or even integral with the cathodetube 31 of the second thyratron needed for the contemplates inverteroperation. Thus, the two cathodes are connected to have commonpotential. As was said above, the two cathode tubes of the twothyratrons are connected to the one output bus from the thermionicconverter, particularly the collector of one of the converter unitsthereof. Suitable high temperature connectors (not shown) providefurther connection between the grids of the cathodes of the thyratronson one hand, and the low temperature grid control device 60, on theother hand. Thyratron 30, which is constructed similar to the thyratron20, has an anode output bus 381 which leads to the other side of theprimary 42 of transformer 40.

As far as the operation of an individual thyratron is concerned, thehigh temperature environment sets the basic operating conditions whichin turn affords the possibility of providing zero drop across anode andcathode electrodes. Turning now to FIG. 3a there is shown the voltageand postulated potential distribution between cathode and anode duringoperation, i.e., after firing of the thyratron. The individual voltagesand potential differences or voltage drops shall be consideredqualitatively at first; I is the work function of the cathode and isequal to the difference in the negative potential of an electron afterhaving left the cathode and the Fermi level thereof. Since an electronloses energy when escaping the cathode the potential of the electronhaving escaped is negative with respect to the potential of the cathodeat Fermi level. For purposes of reference the cathode potential can beconsidered equal to the Fermi level. V, is the voltage drop in theLangmuir or cathode sheath accelerating an electron and raising thepotential thereof when passing through. The sheath drop depends on theionization voltage of the vapor which presently is cesium.

V, is the plasma drop which is comparatively small and depends on thevapor pressure and the distance between cathode and anode in thethyratron. Somewhat larger is the voltage drop V across the gridaperture and resulting from the constriction of the discharge pathbetween anode and cathode by the grid. The plasma drop actually occursat both sides of the grid and thus is divided into two portions, butshould be considered together as it is impractical to distinguishbetween an anode side and a cathode side plasma portion. Therefore, thetotal plasma drop V as shown in FIG. 30 includes the entire plasma ateither side of the grid.

The plasma is essentially macroscopically neutral, i.e., it has not netspace charge. Depending now upon the electric current drawn from theanode during operation, the anode surface may have a positive or anegative space charge sheath or the plasma may extend all the way to theanode as space charge free region. When the electric current actuallyflowing is high there will be a depletion of electrons near the anoderesulting in a positive space charge and a corresponding increase inpotential of the anode, as it is then necessary to pull electronstowards the anode and out of the plasma; this is normally observed inthyratrons.

When the electric current is low there will be also a negative spacecharge in the anode sheath because electrons resulting from thethermionic emission by the anode are added to the plasma electrons, andthe required current does not deplete this space charge. This particularsituation will be prevalent if, as here, the anode surface is large,particularly if it is larger than the cathode surface resulting in alower current density for the anode than for the cathode. Thus, thesheath drops V and V may be of opposite polarity. In particular,electrons passing from the plasma to the anode may have to perform workto reach the anode through the anode sheath, and therefore, they losepotential in accordance with the value V,, providing the current throughthe thyratron is rather low. Finally, a l00 is the work function of theanode.

For establishing zero drop conditions the following relation is to befulfilled: P '-I V,,+V,,+V V,,)=V approximately =0, wherein V, is theeffective cathode-anode voltage drop for the general case. In thisequation, absolute values have been assumed for the several voltages andthe symbol i indicates that the anode sheath drop may have the same orthe opposite direction as the other internal voltage drops in thethyratron combined in parenthesis. The problem is now to adjust thevalues of the several voltage drops in the equation so that V, is atleast substantially equal to zero.

Even though the equation above has quite a number of components as, soto speak, variables, there are several operating factors for thethyratron which determine most of these components regardless of thedesire to produce zero drop across the electrodes. For conventionalthyratrons it is common to select a low cathode work function of a fewvolts so that in accordance with Richardsons Law the current density canbe very high. On the other hand, the ionization voltage of inert gasesused commonly for thyratrons, e.g., xenon or argon and other noblegases, is about 10 volts or even higher, as is the ionization voltage ofmercury. The cathode sheath drop V, is always somewhat below theionization voltage but still rather high for these gases commonly usedfor thyratrons. Thus, for conventional thyratrons D -V, is already anegative value. This precludes zero drop conditions. Moreover, for heavyduty, V,, is negative also and 1 may even be larger than 1 so that it isabsolutely impossible to arrive at V =0.

The result is different if one uses an alkaline metal vapor, preferablyof higher atomic weight, such as rubidium or cesium, with cesiumoffering best performance. The reason why this is so shall be developedlater. The equation above can be interpreted in this manner. The workfunctions 1 and In, are

to be such that their difference at least approximately equals the totalvoltage drop inside of the thyratron and expressed by V,,+V,,,,,+V,V,,.This total voltage drop is to be made as small as possible, because theavailable spread of work functions is, in general, not very large toform an appreciable difference. Thus, the individual components shall bemade as small as possible to establish a small total interior drop inthe thyratron because the first three components all have the same sign.However, the anode sheath drop should be made to correspond to anegative space charge and thus have an opposite sign when compared withthe cathode sheath drop V,.

The two components V, and V are, of course, closely related as they bothdepend on the material chosen for the vapor. Cesium has the lowestionization voltage among the alkali metals and the cathode sheath drop,which is the dominating drop, is accordingly the lowest for cesiumvapor. Cesium is thus the preferred choice. As stated above, the currentthrough the anode determines the extent to which the plasma adjacent theanode is depleted from electrons. In view of the ring configuration ofthe electrodes the discharge path extends radially from the cathode sothat the current density in the anode is lower than in the cathode, eventhough the plasma develops fully also adjacent the anode. In theembodiment shown in FIG. 2 the grid has no grid bars in axial direction;thus, the distinction between a constricted and a spread discharge neednot be made in this embodiment as the discharge is necessarily a spreadone. Therefore, the plasma is developed all around the anode and theanode participates fully in the current conduction.

Presently a low anode current density can be realized due to a spreaddischarge using an anode area which is actually larger than the cathodearea, so that nowhere near the anode is the plasma depleted ofelectrons. It can thus be seen that anode and cathode sheath drops are,in fact, oppositely oriented, so that the total sheath drop V +V,, canactually be made smaller than the cathode sheath drop alone. For cesiumthe cathode sheath drop is between about 0.6 and 1.0 volts, and thetotal sheath drop under the outlined conditions can thus be made to be1.5 volts. However, this may vary with the load ultimately connected tothis power supply system. How this variation could be counteracted willbe also described below.

The plasma drop V is very low, or better, can be made very low if thepd" law is observed. The are drop or plasma drop for a particularmaterial is uniquely related to the product of the vapor pressure p andthe anode-cathode distance d. Even though the minimum of thecharacteristics is not very pronounced, it strongly suggests toconstruct the thyratron so that the product ofa p and dis in a range of10 and 10 Torr mils, to obtain a small arc drop, for example, of theorder of about 0.1 volts. It should be pointed out, however, that inpractice the plasma drop V, cannot be measured, per se, as measuringrequires the introduction of probes inherently providing incorrectmeasuring results. In particular, absolute values for the sheath dropsare not independently measurable from the plasma drop and vice versa.One knows, however, the slope of the plasma drop so that a numericalapproximation of the several values is not pure speculation. However,the region of a minimum plasma drop as a relative value can beascertained by measuring the plasma drop with probes, or even theelectrodes themselves. For cesium, the minimum plasma drop is at about60Torr mils. For obtaining the minimum plasma drop the values forpressure and cathodeanode spacing have to be paired accordingly,regardless of the actual value for the plasma drop.

For structural reasons there is a practical minimum for the electrodedistance, even though miniaturization techniques are well developed inthis field. There is, however, a very real limitation as to the vaporpressure. The deionization time of any vapor is a very importantconsideration for operating a thyratron as a valve, because the plasmamust have decayed before the voltage reversal across anode and cathodereaches appreciable amplitude so that the arc cannot reignite(backfiring).

The permissible deionization time is, therefore, determined by thefrequency of the voltage applied to the thyratron or by the frequencywith which the thyratron inverter is operated as an inverter. For 400c.p.s. the plasma will decay and deionize in between successive halfwaves if the vapor pressure is maintained below about 1 Torr. It will bedeveloped more fully below that, on the other hand, the thermionicemission, i.e., the cathode current density can be increased with vaporpressure, so that two opposing conditions limit the choice of the vaporpressure. The pressure should be as high as possible to achieve maximumelectron emission while still being low enough so that the deionizationtime is short enough to meet the operating frequency requirements. Thus,a pressure of about 1 Torr or of that order of magnitude can be regardedas a more or less fixed parameter.

The recovery time or plasma decay time depends also on the electrodespacing, i.e., it increases with spacing. This is a further conditionfor a small electrode spacing. Operating in, approximately at least, theminimum arc drop region in accordance with the pd law" is compatiblewith this condition. Particularly an electrode spacing of the order of60 mils (at 1 Torr pressure) is consistent with the desire to establisha plasma decay time sufficiently short for inverter operation in atechnically desired frequency range (for example, 400 c.p.s.

It has been proven difficult, however, to provide such a small distancewhen this anode-cathode space has to accommodate both grid and heatshield. This can be circumvented by lowering somewhat the vapor pressureand choosing a somewhat larger electrode spacing accordingly. If onlythe latter is being done, then the plasma drop might increase somewhatbut increase is quite small. However, with reference to FIG. 5 it shallbe discussed that a modified structure does not require a heat shieldand, for that reason, may actually be preferred even though posing otherproblems which shall be discussed more fully below. In conclusion, it isquite possible to construct and to operate the thyratron to establishminimum plasma drop conditions.

Next, the grid drop V has to be determined. It is this a resistancewhich, as stated above, occurs actually in the plasma itself, but whichis not regarded in the usual sense as part of the plasma drop because itresults from a constriction of the plasma by the grip aperture incomparison with the cross section of an electric current path betweenanode and cathode which would be available in the absence of the grid.The constriction is necessary so that the grid can exercise control overthe occurrence (or inhibition) of the thyratron discharge. This, ofcourse, establishes a tendency towards selecting very small gridapertures. On the other hand, a small grid drop requires a large gripaperture. Certainty of control is, of course, the predominant factorhere so that the control conditions must exist without marginal design.A 50 percent grid aperture, referenced against the anode area size orthereabouts, is a workable size for a grid aperture and for providing agrid drop in the plasma of about 0. l to 0.4 volt.

It follows from the foregoing that the total drop V +V,, +V +V rangesbetween .5 and 1.60 volts. That total value is measurable and its valueis, in summary, determined as follows: The anode-cathode spacing and thevapor pressure are chosen to permit sufficient deionization and tooperate at or near the minimum voltage drop in accordance with the pdlaw for plasmas. The choice of the vapor is dictated by the requirementfor a small ionization voltage. All this combined permits operation witha total internal voltage drop of 0.5 to 1.5 volts or thereabouts.

The cathode and anode work functions must now be selected so that theirdifference equals a value in that range. As stated above, the cathodeshould be made preferably of rhenium, having a rather high work functionof about 4.9 volts.

A work function of 4.9 volts places rhenium actually into the class ofthe rather poor electron emitters, particularly if compared with othermaterials used as cathodes conventionally and when operated atcomparably sir nilar temperatures. Moreover, the reason why rhenium isbetter in the present case than, e.g., molybdenum is that rhenium has ahigher work function than molybdenum, which on its face appears to be acontradiction as far as cathode operation is concerned. However, thecontrolling factor for cathode emission of the thyratron in accordancewith the present invention is the interaction between the cathodematerial or cathode substrate and the vapor in the thyratron cavity. Thevapor has been chosen to have a very low work function and a very lowionization voltage for establishing low sheaths drops. That interactionbetween a high work function substrate and low work function-lowionization voltage vapor is crucial for the inventive thyratron and willbe explained next.

The bulk work function of cesium is about 1.8 volts when in the solidstate and somewhat less when in the liquid statev The cathode has atemperature well above the boiling point of cesium so that noprecipitation of cesium on the cathode surface can be expected. n theother hand, the difference in work functions causes cesium molecules toadhere to the cathode surface. This adhesion, therefore, is the resultof interaction between the individual cesium molecules and theparticular cathode surface, and any cesium layer resulting therefrom isnot the result of cohesion between cesium molecules. Of course, due tothe high temperature of the cathode surface cesium atoms will boil away,but new cesium atoms will continuously be caught by the surface fordeposition and adhesion thereon. Thus there is a dynamic equilibriumbetween the adhesion process and the vaporization resulting in a thincesium layer on the cathode. The equilibrium conditions, i.e., thethickness of such layer is dependent on the vapor pressure and on thecathode temperature. The higher the pressure, the more molecules willadhere, but for increasing cathode temperatures the layer will becomethinner. It has been found also that this adhesion is the stronger thelarger the difference between the work functions of the two materials.Since, however, the work function of a gas has no direct meaning, it maybe more accurate to say that the ionization voltage of the gas or vaporshould be rather low, because the ionization voltage of a gas moleculeand the work function of a solid or liquid both represent the ease withwhich an electron can separate from the material, i.e., from theindividual molecule in case of a gas or from the bulk material when inthe solid or liquid state. I

The adhesion, as described in the previous paragraph, in turn modifiesthe effective work function of the cathode. FIG. 3b shows this by way ofexample. The group of upwardly converging curves including, e.g., thecurves A and B, represent Richardsons Law for different work functions,which is one of the parameters in the Richardson equation. The ordinateshows the logarithm of the emission current density and the abscissashows the inverse of the temperature T in degrees Kelvin of the emittingsurface, the scale being drawn to values of l000/T. Richardsons Law hasa proportionality factor as a second parameter which is the same for allcathode materials of interest. However, this proportionality factor isdifferent for cesium and the result thereof will be discussed more fullybelow.

Curve a now is a measured curve and represents the measured emissioncurrent density and dependence upon temperature for a particular vaporpressure and provided that the cathode material, i.e., the cathodesubstrate upon which cesium molecules may deposit is rhenium. This curvea, does not follow the relation given by Richardson's Law, i.e., it doesnot fit into the group of upwardly converging Richardson curves.Instead, it shows a different relationship qualitatively explained bestas follows.

For very high temperatures (to the left of the abscissa) e.g., for 2500Kelvin or thereabouts, very few cesium molecules will adhere to thecathode. Therefore, the cathode is, as far as electron emission isconcerned, modified very little, and the current emission follows theRichardson curve for rhenium rather closely and according to which thecurrent increases with increasing temperatures in a monotonicrelationship.

Thus, curve a, approaches asymptotically the Richardson curve forrhenium. For decreasing cathode temperatures cesium molecules adhere tothe cathode to an increasing extent, and they shield to some extent theotherwise exposed cathode surface, so that the potential barrier of thesolid cathode normally retarding electron escapement from the cathode islowered and the current density decreases with temperature at a rateless than in accordance with Richardsons Law.

For still lower temperatures, say between 2000 Kelvin and 1500 Kelvinthe rate of adhesion of cesium goes up to such an extent that theemission current density now actually increases with decreasingtemperature. Ultimately, for rather low cathode surface temperatures,adhesion of cesium establishes a thick layer of cesium which thendetermines the electron emission with little modification from thecathode substrate. Accordingly, the emission must approachasymptotically the Richardson curve for bulk cesium, i.e., for cesium inthe liquid state. This is to be expected, particularly when the cathodetemperatures approach sufficiently low values with corresponding lowvalues for cesium vapor pressure where the surface coverage of thecathode by cesium is substantially complete. Such surface coverage isdetermined by the equation:

, where: 1 arrival rate of cesium atoms from the vapor n vapor density 6average atom velocity in vapor, and

u. evaporation rate from substrate at given coverage and temperatureconditions The influence of the cathode substrate material on theelectron emission will then be negligible.

Thus, for very low temperatures the Richardson curve for bulk cesium is,in fact, the controlling characteristic for thermionic emission. Thatparticular Richardson curve for cesium is, however, not one of theRichardson curves plotted directly in FIG. 3b because, as was alreadymentioned above, the Richardson emission curve for cesium has a lowerproportionality factor than the high refractory cathode materialscontemplated for use as a cathode substrate, so that in FIG. 3b the lowtemperature asymptotic line for the curve a is a curve preciselyparallel to the plotted Richardson curve having the same work functionparameter (1.8 volts) as cesium but being shifted down therefrom. Inbetween the branch of curve a where the emission current 'densityincreases with decreasing temperature and the low temperature asymptoticportion of curve 0 there is a maximum.

For any given cathode temperature and for a particular vapor pressurethere is now, in accordance with curve a a particular emission currentdensity which is higher than for the substrate-cathode material (e.g.,rhenium) if it had not the cesium layer. The reason for thismodification is twofold. First, due to the cesium layer, the surface ofthe cathode itself is not exposed or is very little exposed directly tothe environment so that the cesium molecules shield the cathode therebylessening the potential barrier of the cathode substrate boundary. Themagnitude of this shielding or reduction in effective surface workfunction is a function of the substrate temperature for a given cesiumvapor pressure. Each point on a, therefore is the measured emission ofthe substrate-cesium combination. The intersection of a with the familyof 'curves A through B yields the work function of the combination. Themaximum in the curve indicates that there is an optimum coverage of thesurface for maximum emission. By varying the substrate temperature, theemission can be varied anywhere between that for the bare substratethrough the emission maximum to an emission level corresponding to bulkcesium. For example, in FIG. 3b a cathode temperature of 1277" C. the

current emission is such (in accordance with curve 11,) that itcorresponds to a work function of 2.3 volts, which is the actual oreffective work function of the cathode at that temperature.

The curves a and a; show similar characteristics respectively for higherand lower vapor pressures but still with rhenium as substrate and cesiumbeing the vapor. The relationship between curves a and a verifies whatwas mentioned above, viz, that the emission current density can beincreased with vapor pressure, and it will be recalled that the requireddeionization time limits the choice of too high a vapor pressure in thethyratron. It will be discussed more fully below how this afiects thechoice of the operating parameter for the thyratron.

The curve b shows the characteristics for cesium vapor at the sametemperature chosen in case for the curve 0,, but with a molybdenumcathode as substrate. One can see that higher emission current densitiesfor similar cathode temperatures and vapor pressures can be obtainedwhen rhenium rather than molybdenum is used as a cathode substrate.Nevertheless, the phenomenon of producing reduced effective workfunctions and higher thennionic emission is present in either case. Inparticular now we observe the inverse relationship for a considerablerange of temperatures. Molybdenum actually has a lower work function andtherefore its emission is higher than the emission of rhenium itself,but the higher work function of rhenium is instrumental in providing alarger attraction for the cesium vapor molecules than molybdenum does,so that for similar temperatures and vapor pressure the effectivethermionic emission is ultimately larger for a rhenium substrate thanfor a molybdenum substrate over a wide range of temperatures. However,it should be repeated that the phenomenon of producing an increasedthermionic emission is present in either case. In other words, usingcesium vapor improves the thermionic emission of both rhenium andmolybdenum but the improvement is better for rhenium than formolybdenum.

To summarize the effect of the vapor, each of the work functionparameter curves such as a,, a b etc., has a high temperature valueasymptotic line which is the Richardson curve for the substrate materialof the cathode. The asymptotic line is, therefore, the same for thecurves a and a i.e., it

. is the rhenium curve. For curve b the asymptotic line is theRichardson curve. for molybdenum. Each of these work function parametercurves 0,, a b etc., passes through a minimum, increases again towards amaximum for a particular temperature, and all these curves dropasymptotically towards the particular Richardson curve for bulk cesium,this relationship being explained here for declining temperatures. Thus,the low temperature asymptotic line is similar to all of these severalcurves and does not depend any more on the substrate. The degree ofindependence is temperature dependent but qualitatively the phenomenonis similar in all cases.

One can see, that over a wide range oftemperatures the current emissiondepends very little on the work function of the cathode substrate exceptthat the work function controls the process of adhesion. The vaportemperature and, more important, a rather large work function differencebetween cathode substrate and vapor material are the dominating factorsfor the high current yield. The temperature regions between minima andmaxima of the work function parameter curves 0,, a b etc., are theprincipal regions of interest here because there are available currentdensities at temperatures much below the temperature for which thecathode substrate material alone would produce comparable thermionicemission. The value of the cathode work function itself is of immediateimportance for the efficiency of the thyratron as proper choice herepermits the establishing of zero drop conditions. For thyratronefficiency however, the current density is also of some importance butonly to the extent that too big a cathode surface, i.e., too big athyratron construction is undesirable in general. In the usual casewhere adhesion does not modify the thermionic emission the work functionof the chosen material for the cathode is a fixed parameter, and thecurrent density can be varied only with the cathode temperature with thehigher density requiring higher temperatures throughout. The presentlyobserved interaction between vapor and cathode material modifies thesimple Richardson relation, in that the work function becomes a variableoperating parameter and both a high thermionic emission and a zero dropcondition can be satisfied.

The selection of the operating parameters to be used yields aconsiderable variety of choices. From the equation above and from FIG.3a, one can see that zero drop conditions require that the anode workfunction be below the cathode work function. By definition the anodeelectron emissivity for its operating temperature must be considerablybelow the corresponding value for the cathode at its temperature,otherwise no proper electric valve operation were possible. Normally, alower work function means higher emissivity except for a drastictemperature difi'erential. However, it is beneficial that for lowtemperatures of a. surface exposed to cesium the emissivity isdetermined by the low temperature asymptotic line of the Richardsoncurve for cesium as stated. Even though the effective work function ofsuch a surface will be rather low, one can see that this is not toocritical because the choice of a rather low temperature for the anodethen benefits from the fact that the proportionality factor for thecesium Richardson curve is lower than the proportionality factor of thehigher refractory cathode materials and this, in turn, results in a verylow anode emissivity in spite of its low work function. Thus, theeffective anode work function will, in fact, be about 1.8 volts, and theanode must cooled to the extent that this work function and thecorresponding low emissivity of such a cesiated anode can be realizedwhich, in turn, makes the anode work function independent from the anodesubstrate and fixes it as a parameter in the equation written above fordefining the condition to establish zero drop across the thyratron.

Here now we turn to the previous result, viz, that for zero dropconditions the anode-cathode work function differential D should bebetween 0.5 and 1.5 volts. This value was determined as a result ofsumming the various internal drops in the thyratron and it is the sumV,,+V,V +V For zero drop condition that sum has to be equal to the workfunction differential. In the previous paragraph we have determined theanode work function to be 1.8 volts as a result of the choice of cesiumfor the vapor material and the anode work function is substantiallyindependent from the anode substrate. It follows, therefore, that thecathode work function has to be a particular value chosen from withinthe range of 2.3 and 3.3 volts in order to establish zero dropconditions and the particular choice, of course, depends on theultimately resulting fixed value for the sum of the several internaldrops.

The parameters to be considered now are, therefore, cathode workfunction, cathode temperature, density of the cathode emission and vaporpressure. The choice of particular values for any two of the fourparameters fixes the values of the other two. The choice of thevariables depends on the requirements and on the desired particulars.The choice is limited, however, by certain constraints, partiallydiscussed above and briefly summarized as follows: The selection of thecathode work function, for zero drop conditions, is particularlycritical. The cathode temperature will be as high as possible, as onecan see from the general nature of the curves in FIG. 3b, but, ofcourse, the temperature is limited by the amount of available thermalenergy, available conditions for cooling and resistance of the variouscomponents including those in the environment against thermaldestruction. The current density should be rather high for reasons ofefficiency, i.e., a set of operating parameters resulting in a very lowcurrent density is of little practical value. The vapor pressure shouldbe high, as follows from the curves of FIG. 3b because there is a directrelation between high current density and high vapor pressure. However,deionization time requirement is severe constraint on the vaporpressure, as is the observation of the pd law because too high a vaporpressure may result in impractically close electrode spacing.

For zero drop conditions the cathode work function is an' ternal drop of0.5 volt can reasonably be expected. A cathode temperature of l320 C.and a current density of about 12 amp/cm are satisfactory values. Theanode temperature being low as above-described, for example, 708 C. Theresulting cesium pressure is above 2 Torrs at a reservoir temperature of289 C. FIG. 30 illustrates the terminal voltage between anode andcathode as a function of interelectrode spacing for these parameters.

For a value of 60 mils this voltage is, in fact, zero. For smallerdistances the system actually generates voltage rather than providingloss. There is a second value, about 3.5 mils where the voltage drop iszero, but this is of little practical significance. The provision of agrid producing a voltage drop V, will, in effect, shift the curve indown direction and for the value of the' grid drop V,, leading tosmaller electrode spacing values. Selection of a smaller vapor pressurepermits an increase of the electrode spacing in accordance with the pdlaw. It is beneficial that a considerable drop in pressure isaccompanied by only a modest drop in current density. Modifying somewhatthe cathode temperature may lead to a different work function for moreconveniently providing zero drop conditions, should the grid drop provetoo large. A larger cathode work function operates in the diagram ofFIG. as shift of the curve in up direction.

As a representative example, for a thyratron it was found highlysuitable to use a current density for about l0 amperes per squarecentimeter. For about 50 percent grid aperture and a load current whichdoes not deplete the plasma with electrons adjacent the anode, theinternal drop, i.e., the required work function differential can be madeas low as 0.5 volts. This results in a cathode work function of 2.3volts and fixes the operating temperature at l277 for a cesium vaporpressure of about 1 Torr, and an electrode spacing of about 60 mils. Theoperating conditions thereby establish assured satisfactory performanceand at a cathode temperature which is not too high and can be realized,particularly in the combination with a converter shown in FIG. 1.

Two other important considerations are to be made. First of all, thegrid should also have a small thermionic emission. The constructionillustrated in FIG. 2 shows that the grid is thermally insulated fromthe cathode through the shield and through the heat chokes. In addition,the grid structure is provided with cooling vanes to cool the grid downto a temperature comparable with the temperature of the anode. This, inturn, means that the low temperature branches of the several curvesillustrated in FIG. 3b are valid also for the grid and the thermionicemission is similar for grid and for anode, which again depends on thefact that the thermionic emission in this cesium filled thyratron cavityis substantially independent from the material chosen for the grid.

A second point to be considered is that the internal voltage drop mayvary with the load current passing through. On the other hand, one cansee from FIG. 3!; that for a given cathode temperature the vaporpressure when varying for any reason also causes a resulting variationin the effective cathode work function. Therefore, by varying theelectric current passing through the reservoir heater 272 the cathodework function can be adjusted. That control in turn may be carried outin dependence upon theload current passing through the anode to therebyestablish a variable cathode work function which follows the variationof the anode sheath drop to maintain zero drop conditions in and acrossthe thyratron.

FIG. 5 illustrates an alternative structure for the thyratron. Thethyratron, as shown here, is a tubular cathode 121 with heat chokes(thin portions) 122 and 123 at the two ends of this cathode tube. Thecathode can be made of any of the materials mentioned above, but againrhenium is the preferred choice. A pair of ring flanges 111 and 113 areused to mount the other electrodes to the cathode tube. In particularthere are ceramic rings 112 and 114 for mounting two grid support annuliand 145 respectively to the flange rings 111 and 113. A plurality ofgrid bars are secured to the two supporting rings 135 and and define agrid cage concentric with and surrounding the cathode tube 121.

By means of two additional ceramic rings 116 and l18, t wo additionalflange rings 125 and 126 are respective mounted to the two annuli 135and 145 for supporting an anode tube 130 which in turn circumscribes thegrid cage. Reference number denotes again the reservoir for cesium, andalso for this embodiment there is provided a heating coil 172 toestablish control conditions for the cesium atmosphere. The liquidcesium is kept sufficiently far from any internal wall facing anddefining the thyratron cavity, so that the liquid cesium can bemaintained at a temperature permitting cesium vapor' to be atequilibrium with its liquid state at a correspondingly low pressure.

The particular structure shown above differs from the one shown in FIG.2 in several important aspects. First of all, the thyratron is longer inaxial direction which enlarges the effec tive cathode emission surface,or, for the same area, a smaller diameter can be chosen. For a givenanode-cathode spacing, this in turn means a relative enlargement of theanode surface in comparison with the embodiment shown in FIG. 3. Therelative enlargement of the anode surface is instrumental in improvingthe anode sheath drop towards zero drop conditions.

Next, the cathode-anode spacing can be made smaller than in the case ofFIG. 2 because no heat shield is interposed between the cathode and theother electrodes. This in turn means that the grid is maintained atfloating heat potential meaning that it has a temperature in between thecathode and the anode temperature. This is a very importantconsideration as it introduces inherently a higher grid temperature intothe i system. In order to counteract any higher thermionic emission fromthe grid the choice of the material for the grid is not open y t H aLooking at curve a,, for example, one can see that for a rhenium surfacenot too much cooler than the cathode, the thermionic emission isactually higher than for the cathode. For purposes of reference curve 0in FIG. 3b represents the thermionic emission for a niobium substrate ina cesium atmosphere under the same pressure and temperature conditionsfor which the curve a for rhenium and b for molybdenum were developed.It can be seen that the current emission of niobium as modified byadhering cesium is well below the emission in accordance with curve a byabout 2 orders of magnitude. Moreover, a rather hot grid is of advantageas in the contemplated range now an increase in the niobium surfacetemperature results in a decrease of the thermionic emissivity as longas one remains in the temperature range between the maximum and theminumum of the curve c,. In other words, the grid should either be ascool as the anode or, if the substrate for the grid is one exhibitinglessv adhesion than rhenium, then the grid temperature should actuallybe not too .xn qtbelq thes t d tsmp ata e v Quite obviously then thegrid temperature can actually be rather close to the temperature of thecathode, which means that. the heat chokes 122 and 123 do not have to bevery pronounced or do not have to be provided for at all. Moreover, onecan see that little provision is required for any active cooling of thegrid nor are any cooling surfaces for the grid structure provided. It isan interesting consideration that the low grid emission obtainable herepermits the thyratron to be operated at low control powers, i.e., a highfiring pulse can be made available at small current in the grid circuit.

The anode 130 has a rather large outer surface because the anode is herean elongated cylinder. This large outer surface therefor provides veryactive cooling.

Another important difference between th e structureslgwn is thepossibility of a constricted discharge due to the bars 140., It has beenfound, however, that for theoperating conditions! expounded above thedischarge is in fact not a constricted one. but a spread one. 7 V V,

The invention is not limited to the embodiments described above but allchanges and modifications thereof not constituting departures from thespirit and scope of the invention are intended to be covered by thefollowing claims.

I claim:

1. A thyratron comprising, a tubular cathode for direct exposure to asource of thermal energy and having a cylindrical outwardly directedemitting surface:

an annular anode having a cylindrical, inwardly directed operatingsurface circumscribing the tubular cathode, and being coaxial therewith,the anode-cathode spacing being exclusively determined by the differencein radii of said anode and cathode surfaces facing each other across thering space between the cylindrical cathode surface and the cylindricalanode surface so that the electron flow, radially outwardly from cathodeto anode, in-.

herently results in lower current density at the anode that firstannular ceramic insulating means for mounting the,

anode in relation to the grid elements;

second annular means including insulating means for mounting the gridelement in relation to the cathode;

means including at least some of the aforementioned elements to define aclosed discharge chamber which contains the said cathode surface, theanode and the grid elements and 3 means for sustaining a particularvapor pressure in the discharge chamber, so that for said chosendifference in radii the plasma drop is about minimum.

2. A thyratron as set forth in claim 1, said second means includingannular heat chokes integral with the cathode.

3. A thyratron as set forth in claim 1, at least one of said cathode,anode and grid elements being made of one of thematerials selected fromthe group which consists of molybdenum, rhenium, tantalum.

4. A thyratron as set forth in claim 1, said cathode being made of oneof the materials selected from the group which consists of rhenium,molybdenum, tungsten, tantalum and nickel.

5. A thyratron as set forth in claim 1 including a plurality of radiatorvanes connected for heat conduction respectively to said anode and saidgrid elements and extending outwardly therefrom.

6. A thyratron as set forth in claim 1, wherein the work functiondifferential of the work functions of said cathode and of said anode isat least approximately equal to the voltage drop resulting from an arcbetween anode and cathode including the combined anode and cathodesheath drops and the drop in the grid space.

7. A thyratron as set forth in claim 1, said tubular cathodecircumscribing a cavity for receiving thermal radiation.

8. A thyratron as set forth in claim 7 wherein a plurality of,thermionic converters are provided, arranged'to define a cav ity forrebEWm radiation, said cavity being contiguous with the cavity ascircumscribed by said tubular cathode, said tubular cathode beingheatedby the radiation which also heats said thermionic converters, furtherincluding elt trEaLQrcuit k 5 nieansfforconnecting the thermionicconverters to said anode,

and cathode of said thyratron.

9. A thyratron as set forth in claim 1, including a reservoircommunicating with said chamber and containing a low boiling pointmaterial in the nonvaporized state.

10. A thyratron comprising, in combination:

a cathode having a particular cylindrical surface for thermionicemission comprising a high refractory material having a relatively highthermionic work function;

an anode having a cylindrical surface and disposed coaxial to thecathode for facing the cathode across a cylindrical ring space;

a grid mounted in spaced relation and coaxial to the anode and to thecathode to establish a controlled discharge path between cathode andanode through the grid;

the active surface of the cathode being spaced from the anode at auniform distance across the cylindrical ring space between coaxial anodeand cathode;

means including the anode and the cathode to provide a closed dischargechamber which includes said discharge P means including a reservoir forintroducing a vapor into the means for providing a temperaturedifferential between the cathode and the anode, including means fordissipating thermal energy from the anode, the anode receivingexclusively thermal energy from the cathode.

11. A thyratron as set forth in claim 10 wherein the vapor pressure isbelow 1 Torr.

12. A thyratron as set forth in claim 10 wherein the vapor is an alkalimetal.

13. A thyratron as set forth in claim 10 wherein said cathode is rheniumor molybdenum, and the vapor is cesium or rubidium.

14. A thyratron as set forth in claim 13 wherein the grid is made ofniobium.

15. A thyratron as set forth in claim 10, said reservoir being mountedto be at a temperature substantially below the cathode temperature.

0 16. A thyratron as set forth in claim 15 comprising in additionheating means for controlling the temperature of the reservoir.

17. A thyratron as set forth in-claim 10, wherein said grid assumes atemperature in between the cathode and anode temperatures and isselected of a high refractory material having a work function below thework function of the cathode material.

18. A thyratron as set forth in claim 10 wherein said anode and saidcathode are made of the same material.

2. A thyratron as set forth in claim 1, said second means includingannular heat chokes integral with the cathode.
 3. A thyratron as setforth in claim 1, at least one of said cathode, anode and grid elementsbeing made of one of the materials selected from the group whichconsists of molybdenum, rhenium, tantalum.
 4. A thyratron as set forthin claim 1, said cathode being made of one of the materials selectedfrom the group which consists of rhenium, molybdenum, tungsten, tantalumand nickel.
 5. A thyratron as set forth in claim 1 including a pluralityof radiator vanes connected for heat conduction respectively to saidanode and said grid elements and extending outwardly therefrom.
 6. Athyratron as set forth in claim 1, wherein the work functiondifferential of the work functions of said cathode and of said anode isat least approximately equal to the voltage drop resulting from an arcbetween anode and cathode including the combined anode and cathodesheath drops and the drop in the grid space.
 7. A thyratron as set forthin claim 1, said tubular cathode circumscribing a cavity for receivingthermal radiation.
 8. A thyratron as set forth in claim 7 wherein aplurality of thermionic converters are provided, arranged to define acavity for receiving radiation, said cavity being contiguous with thecavity as circumscribed by said tubular cathode, said tubular cathodebeing heated by the radiation which also heats said thermionicconverters, further including electrical circuit means for connectingthe thermionic converters to said anode and cathode of said thyratron.9. A thyratron as set forth in claim 1, including a reservoircommunicating with said chamber and containing a low boiling pointmaterial in the nonvaporized state.
 10. A thyratron comprising, incombination: a cathode having a particular cylindrical surface forthermionic emission comprising a high refractory material having arelatively high thermionic work function; an anode having a cylindricalsurface and disposed coaxial to the cathode for facing the cathodeacross a cylindrical ring space; a grid mounted in spaced relation andcoaxial to the anode and to the cathode to establish a controlleddischarge path between cathode and anode through the grid; the activesurface of the cathode being spaced from the anode at a uniform distanceacross the cylindrical ring space between coaxial anode and cathode;means including the anode and the cathode to provide a closed dischargechamber which includes said discharge path; means including a reservoirfor introducing a vapor into the closed chamber, said reservoir beingpositioned remote from the cathode the material of the vapor having aboiling point below the cathode temperature and a work function as wellas an ionization voltage below the work function of the cathode, thevapor pressure being selected so that the plasma drop for a chosen smalldistance between anode and cathode is about minimum; and means forproviding a temperature differential between the cathode and the anode,including means for dissipating thermal energy from the anode, the anodereceiving exclusively thermal energy from the cathode.
 11. A thyratronas set forth in claim 10 wherein the vapor pressure is below 1 Torr. 12.A thyratron as set forth in claim 10 wherein the vapor is an alkalimetal.
 13. A thyratron as set forth in claim 10 wherein said cathode isrhenium or molybdenum, and the vapor is cesium or rubidium.
 14. Athyratron as set forth in claim 13 wherein the grid is made of niobium.15. A thyratron as set forth in claim 10, said reservoir being mountedto be at a temperature substantially below the cathode temperature. 16.A thyratron as set forth in claim 15 comprising in addition heatingmeans for controlling the temperature of the reservoir.
 17. A thyratronas set forth in claim 10, wherein said grid assumes a temperature inbetween the cathode and anode tempEratures and is selected of a highrefractory material having a work function below the work function ofthe cathode material.
 18. A thyratron as set forth in claim 10 whereinsaid anode and said cathode are made of the same material.